Piscirickettsia salmonis is a gram-negative obligate intracellular bacterium that causes systemic septicaemia (salmonid rickettsial syndrome, SRS, or piscirickettsiosis) in salmonid fish. Piscirickettsia—like bacteria are now been recognized with increasing frequency in a variety of other fish species, from both fresh and salt waters around the world. Piscirickettsiosis and piscirickettsiosis-like diseases have affected aquaculture productivity, profitability, the species compatible with commercial rearing, and transportation of fish from site to site. The Chilean aquaculture industry alone attributes annual losses to salmonid piscirickettsiosis of $150 million. In Chile, the syndrome has led to a shift from the more commercially desirable coho salmon to the less desirable but more piscirickettsiosis resistant Atlantic salmon as the primary cultivated species.
Antimicrobials have been tested as a therapy for SRS, but without consistent success. Other suggested measures include attempts to reduce stress in the fish by reducing stocking density, and removing dead fish from tanks without delay. The most practical solution to the SRS epidemic would be to find an effective vaccine to prevent the disease in the first place. Inactivated bacterin preparations from P. salmonis have been shown to have some protective effect, and may be the only suitable option for co-administration in multivalent oil preparations, but are relatively expensive to produce on a commercial scale. Vaccines based on recombinant antigens from P. salmonis have not yet reached the marketplace.
Accordingly, there is an urgent need to make available a vaccine capable of significantly reducing mortalities due to piscirickettsiosis in fish. The present invention is based on the surprising discovery that an existing commercial vaccine product is remarkably effective in preventing the disease. This vaccine is marketed under the name RENOGEN, a live, non-virulent strain of Arthrobacter vaccine. Currently, this vaccine is indicated to protect salmon and other farmed fish against bacterial kidney disease (BKD). The characteristics of this strain are disclosed in WO 98/33884, which is incorporated herein by reference.